The yield in the manufacturing of integrated circuits is affected by many factors. To improve the yield, physical failure analysis (PFA) needs to be performed on at least some of failure dies to find out the root cause of the problems. Since the failure dies may have various different problems, the major failure that affects the yield most needs to be found as a high priority.
The selection of appropriate PFA samples from the failure dies thus becomes critical for catching major failures, and for the process optimization. FIG. 1 illustrates the work flow of a conventional method for selecting PFA samples. First, in step 102, the electrical characteristics of the failure dies are tested, which step is known as a chip probing (CP) step. A CP data log is established to store the results. Next, as shown in step 104, software is used to do a scan diagnosis, in which problems appearing in the failure dies are simulate. If the simulated problems fit the CP data, the simulated problems are identified as being failure candidates. The identified failure candidates in each of the failure dies are provided in the format of coordinates in the respective dies (step 106). For example. FIG. 2 is a schematic top view of an area of a failure die, wherein some metal lines and polysilicon lines are illustrated. Point 120 is the reported coordinate of one of the failure candidates.
To find out the actual location of the problem in the respective failure die, frame 122 (FIG. 2) is drawn with failure candidate 120 as the center, and a manual net tracing (step 108 in FIG. 1) is launched to perform a trace starting from each of the nodes (the black squares) in frame 122. The net tracing results are analyzed to determine the layer the failures occur in (step 110), and engineers may make a manual judgment to the net tracing results. Engineers then select PFA samples (step 112) from those dies whose failures occur at same places (layers). As a result, the selection of the PFA samples is affected by the experience of the engineers. PFA is then performed to the selected PFA samples (Step 114).
If the PFA samples are not properly selected, different PFA samples may reveal different problems that are not convergent. This means that the major failure is not found. Although increasing the number of PFA samples may increase the hit rate of the major failure, it is unrealistic to perform PFA on all failure dies. Conventionally, with the low efficiency of the manual net tracing and the manual analysis, only a very small percent of failure candidates can be analyzed. This significantly affects the accuracy in the selection of proper PFA samples.